The catalyst containing a titanium compound supported on an activated magnesium halide has been known well as a catalyst for preparing an olefin polymer such as a homopolymer of an .alpha.-olefin or an ethylene/.alpha.-olefin copolymer. As such an olefin polymerization catalyst, there has been known the catalyst comprising an organometallic compound catalyst component and a solid titanium catalyst component composed of magnesium, titanium, a halogen and polycarboxylic acid ester as the essential components.
There have already been proposed many methods for preparing a solid titanium catalyst component composed of magnesium, titanium, a halogen and polycarboxylic acid ester as the essential components, such as a method comprising the steps of contacting a solution of magnesium compound with a solution of titanium compound in the presence of an electron donor to give a solid component, carrying the solid component by polycarboxylic acid ester to give a solid product, and contacting the solid product with a solution of titanium compound to produce a solid titanium catalyst component. Further it has been known that the polymer having high stereoregularity can be prepared at high yield by using such a solid titanium catalyst component in polymerization of an .alpha.-olefin of 3 or more carbon atoms.
In such a situation, the present inventors have made an intensive investigation on a solid titanium component more excellent in polymerization activity, and completed the present invention by finding that when a titanium compound mixture liquid which contains a specific amount of hydrocarbon containing halogen-containing hydrocarbon is used as a solution of titanium compound component in preparation of a solid titanium catalyst component according to the process as stated above, the amount of the supported titanium per unit catalyst unexpectedly increases compared with the case of using a 100% purity titanium compound so that a solid titanium catalyst component excellent in activity per unit catalyst can be obtained.
The present invention was made in the light of the prior arts as stated above, and the objects of the invention is to provide a process for preparing a solid titanium catalyst component for olefin polymerization capable of realizing high polymerization activity per unit catalyst, and further to provide a process for preparing a polyolefin using the solid titanium catalyst component obtained by the above process.